In recent years, in order to reduce the weight or cost of parts constituting machines or devices, crystalline plastics such as polytetrafluoroethylene, polyamides, polyethylene, polypropylene, polyacetal, polycarbonate and ABS and thermosetting plastics materials such as polydiallyl phthalate starting from phenol and polymides have come into widespread use in gears, especially those used in a geared motor, etc., bearings, especially roller bearings, cables for automobile speedometers, etc. These plastic materials have superior self-lubricating properties. Their properties are modified by forming composites of these materials, and they are used as sliding members in a dry condition. In order to improve their lubricating properties such as friction and wear, hitherto there have been used lubricating oils such as mineral oil-type lubricating oils, synthetic hydrocarbon oils and ester-type synthetic lubricating oils, and various lubricating greases containing the aforesaid oils as base oils. It has been desired, however, to develop a novel lubricant having a much lower coefficient of friction than the aforesaid conventional lubricants in order to prevent friction, noise, heat generation at frictional parts, etc.
The present inventors made extensive investigations, and consequently found that an imide compound obtained as the product of reaction between a dibasic acid selected from succinic acid, maleic acid, glutaric acid and phthalic acid or the anhydride thereof and a primary amine having a hydrocarbon radical with 8 to 18 carbon atoms exhibits low frictional properties on a sliding surface between non-ferrous materials and a sliding surface between a ferrous material and a non-ferrous material.